Today we are going to talk about RBL servers. I will explain what it is, what it does and how it works exactly.
First, let’s see what it is…
An RBL server is just an abbreviation. RBL means Real-time Black list. Sometimes people also call it DNSBL or DNS black-list.
There are many different types of RBL lists that exist on the Internet and of course, many organizations that support it. The list by itself doesn’t do anything at all. It just collects IP addresses and stores them in the database.
It’s basically a huge list of IP addresses that the owners and maintainers consider to satisfy certain criteria.
Each RBL has different rules for listing IP addresses in their database.
For example, one list may only contain all IP addresses that have an “open relay” server set up on it.
Open relay is just a regular SMTP server who would allow any email to be relayed to anyone without asking for username and password and without checking anything else.
These open relays are routinely found and abused by spammers, so a lot of RBL lists store their IP in their database.
Another example would be a list that contains all servers who violate the SMTP standard as defined by the Internet Engineering Task Force. These servers are deemed less reliable so that not every server would want to receive messages from such servers.
The variety is really wide and services can maintain very different lists, and your IP address can end up being on one of them, especially if your subscribers routinely list your mail as spam by pressing the dreaded “this is spam” button in their mail client.
If enough people complain about your messages, it’s almost certain that your IP will be blacklisted especially by the large providers.
But why is this bad? What’s the problem with being listed in one of those RBLs?
Again, the list by itself doesn’t do much, but other providers may query this list all the time when you want to connect to them.
Now we are getting to the point of how this service works…
It is best described by an example…
Let’s say Joe tried to connect to my server from an IP address 34.76.18.10 and I want to check if this IP address is blacklisted by any RBL.
What I do first is reverse this IP address to make it 10.18.76.34 and append the main sub domain of the RBL list I want to use.
So what I get in the end is: 10.18.76.34.sorbs.com, of course it depends on the RBL list you want to query, and you can query many of them in sequence if you like.
Then you simply send a regular DNS request to resolve this big domain name you ended up with.
Now the RBL server is configured to respond just like any regular DNS server, but with a little twist.
If this IP address is listed, it will respond with a code, for example, it will return: 172.0.0.2.
If it does not respond at all, meaning you do not get the domain name resolved, the IP is clear and not blacklisted.
A simple scheme that works very fast and is built on top of a protocol that is already in use for many years.
Some of the RBL lists can be more sophisticated and can even provide a text reason for a specific IP.
If you just query a TXT type record the same way, some RBL lists will return a text explanation, but usually you don’t need it.
The number of RBLs each server can use is not limited, but since each one must issue a separate DNS query, using too many of them can present a problem.
Big providers avoid this problem, by having an agreement with large RBL providers.
They simply copy their entire list all the time to their local computers. Then those queries become almost instant and do not waste any Internet bandwidth at all.
Today almost all mail providers use at least a few of those lists.
Which lists to choose depends on the trust that each RBL has. If I am going to reject or accept email based on somebody else’s good will, I’d better make sure that this list performs well and does not create a lot of false positives and negatives for me.
So now we get to a very important point…
You as an owner and maintainer of your own SMTP server must make sure that your static IP address is not listed on any of those blacklists, because as you can already tell, if it is, most probably your messages will be rejected in transit.
There are literally hundreds of these lists, but you do not need to concern yourself with all of them because most of them are not very popular and are only used by a few mail providers.
Besides, if you observe a few simple rules, you will always be above the board.
Rule 1 is of course… do not send spam. If you do, no matter how much you try, you will end up on a few of those lists eventually. Most providers know that whenever you send something to a big list, there will be a few people who are always going to be unhappy, and you just can’t do anything about it, but if half of your list starts pushing those spam buttons the moment, you send anything. Your IP is going to end up on a few black lists permanently.
Rule 2 is not to allow your server to become an open relay. You don’t want that to happen, so make sure that only people who have the username, and password can relay email through your server.
Rule 3 is to follow all SMTP standards, which if you are using our SMTP server should be automatic for you.
But what do you do if your IP DOES end up on one of those databases and how do you even know?
To check, you can go to Google and type something like: “multi RBL check”…
You will get a lot of websites that allow you to check many popular RBLs just to make sure if your IP is listed or not. For example, you can use this site:
http://multirbl.valli.org
Simply type your SMTP server’s external IP address into the box and let it rip…
You will normally see a lot of green records. If you see the red ones, check the comments, and you will find instructions on what to do next.
So do not panic but presume that you did something wrong if your IP is listed.
Of course, those lists are just programs, robots and not humans, and obviously, they can make mistakes, but if your IP is listed, assume you did something wrong, because the probability of an error is very small.
Just look at the specific list and find the reason your IP is blacklisted and instructions for delisting your IP.
First and foremost, fix the error or problem that got your IP into the list in the first place.
Keep in mind that most lists provide easy instructions for delisting, and you can easily remove your IP from the list, but if you do it many times without fixing the original problem that got you there in the first place, after some time, most of these lists will refuse to delist your IP.
That’s why I said to fix the original problem first.
RBL servers are a very good addition to your own security as well. If you want to set up your own SMTP server or already own our software, you will find that it is easy to configure RBL servers on our server too.
To see the demo of our program or to get more information, please download a copy of our software below: